Geoff the 5D MKIII was introduced about the time of the bug report now marked as "Implemented." I just found this post by Eric Chan on the Photoshop Family site from six months ago and it appears it isn't "implemented" for older camera models, including the Canon 5D MKIII.

Unfortunately, we can't change the existing 5D3 support (even though technically we know how to do it) because that would post a backward compatibility issue for the images that folks have already shot. In particular, many 5D3 images that have already been cropped by the user in ACR/Lr would suddenly appear to have a different crop, and that would be far worse. My suggestion: If you have a 5D3, don't set the in-camera crop (always leave it to full 3:2 aspect ratio).